[18.x][Docs] Add release note about Clang-defined target OS macros (llvm#80044)

The change is included in the 18.x release. Move the release note to the
release branch and reformat.

(cherry picked from commit b40d5b1)

Author: zixu-w

clang/docs/ReleaseNotes.rst

@@ -168,6 +168,22 @@ Clang Python Bindings Potentially Breaking Changes
   The previous name was incorrect, it was a duplicate of variant 28.
 - Refactored enum implementation, switching to the standard library `Enum` type.
 
+Clang Frontend Potentially Breaking Changes
+-------------------------------------------
+- Target OS macros extension
+  A new Clang extension (see :ref:`here <target_os_detail>`) is enabled for
+  Darwin (Apple platform) targets. Clang now defines ``TARGET_OS_*`` macros for
+  these targets, which could break existing code bases with improper checks for
+  the ``TARGET_OS_`` macros. For example, existing checks might fail to include
+  the ``TargetConditionals.h`` header from Apple SDKs and therefore leaving the
+  macros undefined and guarded code unexercised.
+
+  Affected code should be checked to see if it's still intended for the specific
+  target and fixed accordingly.
+
+  The extension can be turned off by the option ``-fno-define-target-os-macros``
+  as a workaround.
+
 What's New in Clang |release|?
 ==============================
 Some of the major new features and improvements to Clang are listed
@@ -501,6 +517,15 @@ New Compiler Flags
 - ``-fpointer-tbaa`` enables emission of distinct type-based alias
   analysis tags for incompatible pointers.
 
+.. _target_os_detail:
+
+* ``-fdefine-target-os-macros`` and its complement
+  ``-fno-define-target-os-macros``. Enables or disables the Clang extension to
+  provide built-in definitions of a list of ``TARGET_OS_*`` macros based on the
+  target triple.
+
+  The extension is enabled by default for Darwin (Apple platform) targets.
+
 Deprecated Compiler Flags
 -------------------------